Types and distributions of porosities of sandstones of the Cretaceous Maceió Member, Sergipe-Alagoas Basin, northeastern Brazil

Abstract

The porosity and permeability of the sandstones of the Lower Cretaceous Maceió Member of the Muribeca Formation were studied in 51 cores taken from the northeastern half of the Sergipe-Alagoas Basm, located on the Brazilian passive continental margin. The Maceió Member is a feldspathic sandstone that thickens offshore to more than 5 000 m. The Maceió includes seven facies ranging from alluvial to submarine fans in the upper part and includes lacustrine and evaporite environments 10 the lower part. One hundred and thirteen thin sections from plugs were used to systematically quantify 12 pore types and sizes and to identify a diagenetic sequence of 13 mineral phases. Stepwise regression helped to Identify the best predictors of plug permeability: intergranular clean pores, intergranular pores displaying residual sol/ds, and intergranular open pores displaying carbonate cement. The performance of over 380 clay mineral analyses contributed to the understanding of diagenetic history. The most important conclusions reached include the following: Porosity and permeability are best at the top of the Maceió Member, while the highest values are observed where the Maceió is shallow and near its outcrops. Intergranular secondary porosity is predominant at the top throughout the Maceió and is the best predictor of permeability. Most of this intergranular porosity was formed by the invasion of meteoric walers, especially in the shallow Maceió. However, minor porosties were possibly formed by organic acids in the deeply buried Maceió.